Deep neurons in piriform cortex. I. Morphology and synaptically evoked responses including a unique high-amplitude paired shock facilitation.

1. Synaptic responses of cells in layer III of the piriform cortex and the subjacent endopiriform nucleus (layer IV) were analyzed with intracellular recording techniques in a slice preparation from the rat, cut perpendicular to the pial surface. 2. Micropipettes containing Lucifer yellow (LY) were used to correlate response properties with morphology. An antiserum to LY was used to intensify staining and to prevent fading during detailed morphological study. Response properties were also examined with potassium acetate-containing electrodes. 3. Morphologically, two cell types were identified: pyramidal cells that were confined to layer III of the piriform cortex and multipolar cells that were in layer III and the endopiriform nucleus. 4. In morphology, deep pyramidal cells in layer III closely resembled superficial pyramidal cells in layer II, with the exception that primary apical dendritic trunks were longer and basal dendritic arborizations were more extensive than apical. Like superficial pyramidal cells, apical dendrites of all deep pyramidal cells stained extended through the afferent fiber termination zone in layer Ia and gave rise to local axonal arbors that were concentrated in layer III and the endopiriform nucleus. 5. Multipolar cells were morphologically indistinguishable in layer III and the endopiriform nucleus. All gave rise to nonvaricose spiny dendrites that never extended into layer II and local axonal arbors. 6. Response properties of deep pyramidal and multipolar cells were similar; responses of both of these populations were very different from those of superficial pyramidal cells. The primary difference between responses of deep pyramidal and multipolar cells was a shorter latency of postsynaptic potentials evoked in deep pyramidal cells by stimulation of afferent fibers, consistent with the extension of their dendrites into layer Ia. 7. Responses of most deep cells to stimulation of afferent and association fibers at sufficiently high strength consisted of an initial excitatory postsynaptic potential (EPSP), followed by a fast Cl- -mediated and a slow K+-mediated inhibitory postsynaptic potential (IPSP). 8. A characteristic feature of deep cells, which was only rarely observed in superficial pyramidal cells, was the presence of variable EPSPs evoked at long latencies (greater than 100 ms) by stimulation of afferent or association fibers. 9. A striking finding for deep pyramidal and multipolar cells, when studied with LY-containing pipettes, was a variable slowly rising depolarizing potential triggered at depolarized membrane potentials by stimulation of afferent or association fibers.(ABSTRACT TRUNCATED AT 400 WORDS)